1. Field of the Invention
The present invention relates to an electron emission cathode; an electron emission device, a flat display, and a thermoelectric cooling device incorporating the electron emission cathode; and a method for producing the electron emission cathode.
2. Description of the Related Art
In recent years, research directed to realizing high-performance devices, such as ultra-high speed devices, by integrating minute electron emission devices using semiconductor miniaturization techniques has been vigorously conducted. This field of research is referred to as "vacuum microelectronics". Vacuum microelectronics has been attracting particular attention to its applications for flat displays (or field emission displays; hereinafter referred to as "FEDs") because the use of electron emission devices for a FED is considered to lead to a thinner and lighter display device than conventional cathode ray tube displays.
In a FED incorporating electron emission devices, the electron emission devices are disposed in a two-dimensional arrangement so as to oppose an anode on which a fluorescent substance is applied. By applying a voltage between each cathode and the anode, electrons are drawn out into a vacuum, where the electrons collide with the fluorescent substance so as to be excited and emit light.
Hereinafter, a conventional electron emission device will be described. In general, a current density J, when drawing out electrons from a solid electrode in a vacuum, is derived in accordance with Fowler-Nordheim's formula (Eq. 1): EQU J=(A.multidot.F.sup.2 /.PHI.).multidot.exp(-B.multidot..PHI..sup.3/2 /F)eq. 1
In the above equation, A and B represent positive constants; F represents an electric field; and .PHI. represents a work function of the cathode. Assuming that a voltage V is applied when drawing out electrons, the electric field F is derived in accordance with eq. 2: EQU F=.beta.V eq. 2
In the above equation, .beta. is a constant which is determined by the geometrical shape of the cathode.
In accordance with eq. 1 and eq. 2, the current density J can be increased, while keeping the applied voltage V constant, by increasing .beta. and/or decreasing .PHI.. However, in the case where a semiconductor is used for the cathode, the current density J can be increased by decreasing an electron affinity .chi. (which is the difference in energy between a vacuum level and the conduction band of the semiconductor) instead of the work function .PHI.. In order to increase .beta., it is necessary to process the cathode so as to have a sharp point. Specifically, a method is often taken which etches an n-type silicon substrate so as to form an electron emission portion having a sharp-pointed projection, for example.
FIG. 18 is a schematic cross-sectional view showing a conventional electron emission device having a sharp-pointed projection as an electron emission portion. As shown in FIG. 18, the electron emission device 500 includes a silicon substrate 504 having an electron emission portion 502 and a gate electrode 508 formed on the silicon substrate 504, with an insulating film 506 interposed therebetween, so as to surround the electron emission portion 502. The electron emission portion 502, which has a pointed conical shape, is obtained by etching the silicon substrate 504. An electrode 510 is provided on the silicon substrate 504.
By placing the electron emission device 500 in a vacuum so as to oppose an anode and applying a positive voltage of several dozen volts to several hundred volts to the gate electrode 508 with respect to a potential of the silicon substrate 504, an electric field concentrates at the electron emission portion 502 because of its pointed tip. Then, a potential barrier formed by a vacuum level is lowered for the electrons in the electron emission portion 502, and the potential barrier becomes thinner, so that electrons are drawn out into the vacuum from the surface of the electron emission portion 502 owing to a tunnel effect. The electrons thus drawn out are captured by the anode opposing the silicon substrate 504, a positive voltage of several hundred to several kilo volts being applied to the anode with respect to a potential of the gate electrode 508.
In the case of an electron emission cathode composed only the silicon substrate 504 and the electron emission portion 502, without any gate electrodes included, electrons are directly drawn out and captured by an opposing anode when a voltage of several hundred to several kilo volts is applied between the anode and the silicon substrate 504.
As a conventional thermoelectric device for converting electric energy into thermal energy, a thermoelectric cooling device 520 shown in FIG. 19 is known. The thermoelectric cooling device 520 has a structure in which n-type semiconductor layers 522 and p-type semiconductor layers 524 are alternately connected to one another in series via metal plates 526 and 528. By applying a voltage between terminals 530 and 532, either the metal plates 526 or the metal plates 528 are cooled, while the other metal plates 526 or 528 are heated.
However, the above-mentioned conventional electron emission device has the following problems.
First, the tip of the electron emission portion must be processed with an accuracy on the order of nanometers, thus requiring highly sophisticated semiconductor processing techniques. Therefore, it is difficult to produce FEDs incorporating such electron emission devices at a low cost. Moreover, the shape of the tip of the electron emission portion tends to vary, thereby resulting in a nonuniform display by the FED. Furthermore, the tip of the electron emission portion is likely sputtered by ion particles colliding in a vacuum, thereby resulting in degradation of the tip within a rather short time period. As a result, it is not expected that a FED having a long lifetime can be realized.
In the case where a FED is constructed by using the above-mentioned electron emission device, it is necessary to realize a vacuum on the level of about 10.sup.8 to 10.sup.10 Torr. Commercial production of FEDs using such a vacuum is not realistic.
Furthermore, a silicon substrate is used as the substrate, which limits the size of the display size of the FED. This leads to the problem of difficulty in realizing large-display FEDs.
In a conventional thermoelectric cooling device, metal plates to be cooled and metal plates to be heated are connected to one another via the n-type semiconductor layers 522 and the p-type semiconductor layers 524. Heat is transmitted from the plates to be cooled to the plates to be heated via these semiconductor layers 522 and 524, thus resulting in a large leakage of heat. This results in a very low cooling/heating efficiency.